Etapas de desarrollo profesional: concepciones y caracterización

The results for the two human and four bovid teeth are shown in Figure 5-23 and Table 5-11. Since dentine and enamel form at the same time, we would expect them to show the same 87_Sr/86_{Sr isotope}

ratio. The observation that all samples show lowered 87_Sr/86_{Sr isotope ratios in their dentine compared}

to the enamel indicates, that the dentine has been partially overprinted with the 87_Sr/86_{Sr isotope range}

of the site.

The bovid enamel samples show 87_Sr/86_{Sr isotope ratios of 0.71152±17, 0.71197±14, 0.71169±74 and}

0.71092±22. These values are on the upper limit of isotope package 2 and fit well within the highly variable clastic sediment units of the Aquitaine and Paris Basin as defined by isotope package 3. They are below the 87_Sr/86_{Sr isotope ratios expected from many units within the Massif Central and the}

Armorican Massif. This indicates that the movement of the bovids was constrained to areas dominated by carbonaceous and clastic sediments in the Aquitaine and Paris Basin (Figure 5-22B). The lower

87_Sr/86_{Sr isotope ratios of 0.71092±22 suggest that this bovid had a different feeding area than the other}

three, most likely within the Basins.

The human enamel 87_Sr/86_{Sr isotope ratios are 0.71351±41 and 0.71332±14 for sample 3455 and 3456,}

respectively. These values are higher than the carbonaceous and clastic sediments of the Aquitaine and Paris Basins, excluding these regions as possible childhood residence areas. They are also lower than isotope package 5 (orthogneiss) units. The closest fit for their 87_Sr/86_{Sr isotope ratios are the schist units}

150

these units is 12 km to the east of the site (Figure 5-22B). The granites and metamorphic rocks of that dominate the Massif Central and Armorican Massif are highly variable in their 87_Sr/86_{Sr isotope ratios}

(isotope package 4) and are thus also possible childhood residence areas. In conclusion, the two individuals did not spend their childhood in the immediate vicinity of the burial site but come from an area with higher 87_Sr/86_{Sr isotope ratios. In terms of distance the closet possibly match is the western}

part of the Massif Central. Additional tracers such as oxygen and lead isotopes could be used to further constrain the childhood residence area for these individuals.

Figure 5-22: A: Surface geologic map of France (BRGM). B: 87_Sr/86_{Sr isotope packages, data taken}

151

Figure 5-23: 87_Sr/86_{Sr isotope ratios for the human and bovid samples from la Grotte des Perrats. The}

dashed lines indicate the 87_Sr/86_{Sr isotope range for the lithological units. In addition to the isotope}

packages we added a further subdivision to distinguish between clastic sediments in the basins, and clastic sediments in the Massif Central.

Table 5-11: Results for the human and bovid samples from la Grotte des Perrats.

Sample ID 88_{Sr Volts} 87_Sr/86_{Sr ±2se}

Human 3455 Dentine 2.85 0.71174 0.00003 Enamel 1.85 0.71351 0.00041 3456 Dentine 1.16 0.71163 0.00016 Enamel 0.97 0.71332 0.00014 Bovid 3557 Dentine 0.81 0.71046 0.00017 Enamel 0.29 0.71152 0.00045 3558 Dentine 1.39 0.71112 0.00014 Enamel 1.19 0.71197 0.00012 3559 Dentine 1.06 0.71054 0.00008 Enamel 0.75 0.71092 0.00022 3560 Dentine 0.51 0.71078 0.00021 Enamel 0.23 0.71169 0.00074 Marine Dugong 16.73 0.70920 0.00001 Dugong 21.78 0.70919 0.00001 Shark 17.46 0.70918 0.00002 Shark 13.99 0.70921 0.00002

152

6

Conclusions and future directions

The aim of the research project was to (i) define the bioavailable strontium isotope range of France, (ii) develop least-destructive analytical techniques to screen for diagenetic overprint and to perform strontium isotope analysis of fossil human teeth, and (iii) to apply isotopic tracing to key archaeologica l sites in France. The main research outcomes and future directions for each of these aims is discussed below.

(i) The IRHUM database (Willmes et al., 2014) and the new baseline map of the spatial variability of bioavailable strontium isotopes for France (Willmes et al., in review), provide a powerful tool for archaeological provenance studies. This database and map are also potentially suitable for other applications such as food provenance studies and forensics. The database (www.irhumdatabase.com) and its associated dataset are both open access. Usage of the open source software GeoNode (Boundless) for the database and storage of the dataset in the Pangea data repository (doi:10.1594/PANGAEA.819142) proved to be a fundamental advantage, by allowing for easy access and future developments from the scientific community.

(ii) Using the plant and soil samples from the IRHUM database we produced the first bioavailable 87_Sr/86_{Sr isotope baseline map for archaeological provenance studies in France.}

Significant differences in 87_Sr/86_{Sr isotope ratios were observed between plant samples and}

soil leachates at a number of sample locations. Identification of the driving process behind these differences is confounded by the complex interplay between weathering of lithology, soil genesis, plant processes, and external strontium inputs that vary both in absolute strontium concentrations as well as isotope ratios, spatially and with time. Based solely on the strontium isotope ratios it is thus not possible to untangle these processes and quantification of these external strontium inputs was beyond the scope of this work. For future sampling campaigns, it would be beneficial to collect a number of soil and plant samples from a single site to investigate the full range of the different Sr fluxes between soil horizons, plant types, and external Sr inputs, over multiple seasons.

To create a robust baseline map, we incorporated the observed local variability but excluded anomalous sites that are not representative of their lithological unit and geographic area. Removing these samples sites did not influence the overall variability of the lithologic a l units, with the exception of gravel and chalk units, and thus represents a viable approach.

87_Sr/86_{Sr isotope ranges for all major lithological units were established. These were then}

grouped into five isotope packages, based on k-means cluster analysis, to achieve minimal internal variability and maximise the difference between the isotope packages. The large

87_Sr/86_{Sr isotope ranges found in many lithological units and isotope packages, and the}

occurrence of similar lithological units with overlapping 87_Sr/86_{Sr isotope ranges at}

153 areas. In addition, the use of modern samples to create a map for archaeological provenance studies may not be appropriate if the surface deposits have changed significantly (e.g. deposits from the Ice Ages) or if the climatological and atmospheric conditions were different enough to significantly change the 87_Sr/86_{Sr isotope ratios in plants and soils.}

Nevertheless, keeping the limitations of this map in mind it still provides a useful tool to identify patterns of mobility within France and to identify areas suitable for more in-depth studies of strontium isotopic tracing. A logical next step is to create isotope variability maps for additional isotopic tracers such as oxygen and lead. Combining multiple independent proxies would greatly improve the ability to discriminate different geographic regions. In addition, strontium isotope maps of the Iberian Peninsula and Italy would greatly enhance the current capabilities of strontium isotope tracing in Europe.

(iii) The isotopic analysis of valuable fossil human remains requires the use of least destructive analytical techniques. Systematic mapping of U, Th, Zn, and Sr element distributions can detect diagenetic overprint in fossil teeth, which is a common problem in archaeologica l studies. Comparison of a modern human tooth and a Neanderthal tooth showed the full potential of these chemical tracers to detect zones within a tooth, which are least affected by diagenetic overprint. This nearly destruction free screening method ensures that only suitable samples are further processes for either micro-drilling TIMS or in situ LA-MC- ICP-MS analysis.

The polyatomic interference on mass 87 is the principal cause of the offsets between solution and LA-MC-ICP-MS strontium isotope analyses observed in a significant number of analytical facilities. We found direct evidence that this interference originates from Ar, rather than Ca. The effect of the interference on the 87_Sr/86_{Sr isotope ratio is essentially}

controlled by the Sr concentration and oxide production rate. This is because both Ca and P are stoichiometric components in bioapatite, and Ar is always present in the plasma. Currently, monitoring for this interference and, if present, reducing the oxide production rate is the most promising approach to minimise the effect of this interference. The improved analytical protocol significantly reduced the effect of the polyatomic interference on the 87_Sr/86_{Sr isotope ratio to 38±394 ppm (n=21,} 1σ). _{In terms of mobility studies, the}

analytical uncertainties of LA-MC-ICP-MS analysis are now significantly smaller than the variability in 87_Sr/86_{Sr isotope ratios in the environment, thus allowing the use of in situ LA-}

MC-ICP-MS as an alternative to micro-drilling TIMS.

(iv) Combining rapid screening for diagenetic overprint and least destructive strontium isotope analysis, either by in situ LA-MC-ICP-MS, or micro-drilling TIMS, has opened the door to previously unavailable human fossil samples.

The study at Moula-Guercy of 2 Neanderthal individuals and one Neolithic individual was part of a larger effort to investigate the chronology and mobility of this important

154

Neanderthal site. The radiometric chronology for Moula-Guercy, provided by combined U- series/ESR dating, indicated that the crucial layer XV, corresponds to MIS 5 sensu lato, and is younger than MIS 6. These results agree with the biostratigraphy at the site, which places layer XV to MIS 5e. Strontium isotopic tracing on two Neanderthal individuals provided consistent results with the archaeological data. This shows that the Neanderthals were mobile within the Rhône Valley and used Moula-Guercy as specialized, short-term hunting camp, when their prey animals, such as red deer, were seasonally abundant in this area. Care should be taken to extrapolate from this small (n=2) dataset to any interpretation of Neanderthal mobility overall and more direct investigations of Neanderthal mobility are certainly needed. In contrast, the Neolithic individual showed strontium isotope ratios indicative of young volcanic units. The closest volcanic units occur ~30 km south east of Moula-Guercy, but a more likely residence area for this individual are the much larger occurrences of these units in the Massif Central ~50-80 km to the west of the cave. In either case, this individual experienced significant mobility in its lifetime. To gain a better understanding of Neanderthal mobility as a whole, least destructive strontium and oxygen isotope studies could be carried out on a range of Neanderthal samples across France.

The study at Le Tumulus des Sables, an important Neolithic site near Bordeaux, was the focus of a new radiocarbon dating campaign combined with multi-isotope study to reconstruct mobility and diet using strontium, oxygen (James et al., 2013), nitrogen and carbon isotopes. We found that the site was used for burials over a much longer period than previously thought. Initially classified as an early Bell Beaker site, the radiocarbon chronology and artefacts associated with the burials instead document occupation over a period of ~ 2.5 ka. Given the tendency of the BBP people to re-use collective tombs of preceding cultures, and considering the relatively large quantity of Bell Beaker material exhumed (especially ceramic finds), it is possible that most of the human remains found at this site represent the deceased of the BBP. However, without dating each individual the disturbed condition of the burial does not allow the secure identification of a specific individual to the BBP. The remains of eight individuals had narrow C and N isotopic ranges, suggesting that the diet of the inhabitants remained terrestrial throughout the lifetime of the site, despite its close proximity to the Gironde Estuary and the Atlantic Ocean. Assuming a local, terrestrial diet, the geographic differences in bioavailable 87_Sr/86_{Sr and precipitation} δ18_{O within southern France make it possible to infer the regions in which the individua ls}

spent their childhood. Five groups were identified that represent statistically different isotopic compositions of their food and drinking water source. Interpreting these isotopic differences in terms of human mobility is limited due to the large range of bioavailab le

87_Sr/86_{Sr near Le Tumulus des Sables and in the surrounding Aquitaine Basin and the}

uncertainties associated with interpreting the δ18_O

W values. Nevertheless, for three of the

155 as locals to the site and individuals from group 2 and 3 as possible short-distance migrants from the surrounding Aquitaine basin. The study at La Grotte des Perrats provided evidence that the individuals found at this site may have come from the Massif Central. This could have implications for the interpretation of cannibalism at this site, but only presents one line of evidence and needs to be substantiated with more archaeological and geochemical datasets. The different archaeological case studies illustrate the potentials and pitfalls of strontium isotope tracing, and offer new insights into these renowned archaeological sites. Especially when multiple isotopic tracers are used in conjunction with archaeologica l evidence, detailed insights about the mobility patterns of our ancestors can be gained from minute amounts of sample material.

156

7

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